1. Field of the Invention
The present invention relates to a process for the catalytic oxychlorination of ethylenically unsaturated hydrocarbons. The catalysts used in the process are designed for use in fluidized bed reactors, and are capable of providing high conversion rates and high selectivity, while avoiding problems due to stickiness.
Oxychlorination catalysts are well known. The most common oxychlorination reaction is the conversion of ethylene to ethylene dichloride (dichloroethane). Ethylene dichloride (EDC) is an intermediate to vinylchloride, which itself is the monomer for polyvinylchloride (PVC).
The oxychlorination reaction of ethylene is described by the following equation:C2H4+½O2+2HCl→C2H4Cl2+H2O
Various options are available for carrying out this reaction. Air may be used as the source of oxygen, or oxygen, either by itself or mixed with an inert gas, may be used. Also, the reaction may be carried out in a fixed bed or in a fluid bed reactor. Fluid bed reactors are preferred.
Fluid bed reactors use catalysts based on copper salts, preferably CuCl2, on a support. The support has a particle size suitable for good fluidization. The support particles may consist of an alumina or various alumina silicates, with alumina being the preferred support material, gamma-alumina being most preferred.
In addition to copper salts the catalyst may contain salts of alkaline metals, alkaline earth metals, and rare earths.
A serious problem with fluidized bed catalysts of this kind is caused by a phenomenon referred to as “sticking”. This term describes the propensity of catalyst particles to stick to each other. It will be appreciated that, when this happens on any significant scale, the fluid properties of the bed are disturbed. At the same time the free-flowing properties of the catalyst in cyclone devices used for separating entrained catalyst particles from the product stream are negatively affected. This results in significant amounts of catalyst being removed from the reactor by the flow of gases. Catalyst material thus entrained by the gas flow ends up in the quench unit that is installed downstream of the reactor. An increased amount of catalyst material in the cyclones is a tell tale sign to the alert operator that sticking might be occurring.
Sticking also leads to a reduced reaction rate, and thus a reduced conversion of reactants, which is evidenced by an increase in residual HCl. An increase in residual HCl in turn causes sticking to increase, which risks putting the reaction conditions into a vicious cycle.
An alert operator may be able to reverse the occurrence of sticking, provided it is detected early enough, by increasing the oxygen feed ratio and/or the reaction temperature. Both actions result in a higher conversion rate, which lowers the amount of residual HCl, but also results in increased oxidation to CO and CO2 (jointly referred to as COx). Thus, the measures required to reverse sticking or to avoid sticking cause the product yield to decrease, and therefore carry a considerable cost.
Oftentimes it will not be possible to reverse an occurrence of sticking by adjusting the reaction conditions. It may be necessary to stop the reactor, clean it out, and start it back up. It will be appreciated that the phenomenon of sticking may cause serious damage to the economic operation of an oxychlorination reactor.
Although the phenomenon of sticking has been the subject of much discussion and many publications, it continues to be poorly understood. It is generally accepted that it is the copper in the catalyst that is responsible for “stickiness”, that is, the propensity of catalyst particles to stick together. During the catalytic reaction the copper ions are reduced from Cu(II) to Cu(I), and again oxidized to Cu(II). It is generally believed that copper in its Cu(I) form is the main actor in the sticking phenomenon. The theory is that Cu(I) compounds, such as CuCl, are mobile or even liquid at the reaction temperature. According to this theory these mobile copper compounds migrate to the surface of the support particles and form liquid bridges with other such particles.
Based on this theory it has been suggested to form oxychlorination catalysts by co-precipitation of Cu ions and alumina. The copper ions in a co-precipitated catalyst are more strongly held in the alumina matrix than copper ions deposited on an alumina support by impregnation. One would expect co-precipitated catalysts to be less susceptible to sticking, and this is indeed the case. However, co-precipitated catalysts are less active than catalysts obtained by impregnation because the copper ions are less accessible to the reactants.
2. Description of the Related Art
EP 0 119 933 B1 discloses a different approach to reducing stickiness. The goal is to prepare an impregnated catalyst consisting of porous particles of which the surface layers are relatively poor in copper content. This is accomplished by also impregnating a solution of MgCl2, and carrying out the impregnation in the presence of a strong acid. The copper content of the outer layers of the particles is determined by X-ray photoemission spectroscopy (XPS).
Because of the mobility of copper compounds it is unlikely that XPS could provide a meaningful picture of the presence of copper on porous support particles. Even if such a thing could be measured at ambient conditions, the result is of limited validity for the situation under reaction conditions, when the much more mobile Cu(I) compounds are formed and the temperature is much higher. If indeed the copper ions are preferentially located in the pores, away from the surface, one would expect the activity of the catalyst to suffer, as the copper ions are less available to partake in the catalytic reaction.
Yet another approach has been the use of promoters to increase the activity of the catalyst. U.S. Pat. No. 4,069,170 discloses supported catalysts comprising salts of copper, potassium, didymium (a mixture of rare earth metals, rich in La) Lanthanum, and optionally magnesium. The catalyst composition “does not cake and does not cause defluidization” (see Abstract of US '170). Although the disclosed catalyst composition contains from 0.5% to 15% CuCl2, the specific examples without exception contain 5.3% or less CuCl2. Also, all specific examples contain an amount of DiCl3 that exceeds the amount of CuCl2. In addition, these catalysts contain significant amounts of KCl and LaCl3. Apparently, the approach taken is to reduce stickiness by lowering the amount of copper in the composition, and to compensate for the resulting loss in catalytic activity by adding significant amounts of promoter materials.
In spite of these efforts there continues to be a need for oxychlorination catalysts having copper as the main catalytically active metal, wherein the copper ions are fully accessible to the reactants, and that do not suffer from stickiness.
The problem of stickiness is particularly acute with catalysts designed for use in fluidized bed reactors comprising baffles. These baffled reactors differ from a conventional fluidized bed reactor in that they contain a number of perforated plates, placed at different heights in the reactor. As compared to conventional fluidized bed reactors, the residence time of the reactants in a baffled reactor is much shorter. This much shorter residence time requires a catalyst having a high activity. This high activity is typically attained by providing a catalyst having a relatively high copper loading. The preferred catalyst has the copper introduced to the catalyst by impregnation, rather than by co-precipitation with the support material. Using impregnation as the technique for introducing copper into the catalyst increases the activity of the catalyst, but also increases the risk of stickiness.
It is therefore an object of the present invention to provide a process using a copper-based oxychlorination catalyst having a high activity, while not being susceptible to stickiness. It is a further object of this invention to provide a catalyst suitable for use in a baffled reactor, without being susceptible to stickiness. In a preferred embodiment the catalyst is made by impregnating a support material with a solution of CuCl2.